The idea of ?international law? is highly linked to the thought of Hugo Grotius, a Dutch lawyer from the first part of the 17th century and leading thinker of the concept of ?international society?. Grotius was a major theorist of the Renaissance in Europe and his reflection had a significant influence on the Treaties of Westphalia (1648), resulting of the ?Thirty Years' War? among Catholic and Protestant European nations. But the core principle of his work is without doubt the implementing of international law (called ?the law of nations? in his day), referring to the rules governing international relations, and its relationship to the universal concept of ?natural law?. As a matter of fact, Grotius conceives natural law as a presentation of right reason according to which we necessarily judge an action to be just or immoral depending on its conformity with reasonable and social nature.

This concept is not new and was first developed by the religion of the Stoic and the Aristotelian thinking during the Ancient Greece. Nonetheless, it seems important to emphasize on the fact that despite being the ?father? of natural law theory as it is wrongly assumed; Grotius is however best-viewed as the principal thinker of the Protestant natural law which arose in the 17th century . Since then, his weight on the evolution of European law over the last three centuries is sizeable and core in the European vision of international law; even though his thought has also encountered much criticisms and may be put into perspective.